There has been explosive growth in the area of wireless communications. A few years ago, the sight of a person speaking into a cellular phone was a curiosity; now it is commonplace. Communication via cellular phones is supported by wireless telecommunications systems. Such systems service a particular geographic area that is partitioned into a number of spatially-distinct areas called "cells." Each cell usually has an irregular shape (though idealized as a hexagon) that depends on terrain topography. Typically, each cell contains a base station, which includes, among other equipment, radios and antennas that the base station uses to communicate with the wireless terminals (e.g., cellular phones) in that cell.
The antenna used for transmitting signals from a base station is typically a linear phased-array antenna. A phased-array antenna is a directive antenna having several individual, suitably-spaced radiating antennas, or elements. The response of each radiating element is a function of the specific phase and amplitude of a signal applied to the element. The phased array generates a radiation pattern ("beam") characterized by a main lobe and side lobes that is determined by the collective action of all the radiating elements in the array.
It may be desirable, at times, to adjust the geographic coverage of a particular base station. This can be accomplished changing the azimuth ("beam steering") or elevation ("beam tilting") or both (henceforth "beam steering"), of the beam generated by a base station's transmit antenna. The beam generated by a linear phased-array antenna can be steered by employing a progressive element-to-element phase shift.
A signal for transmission is delivered to the phased array via a corporate or a series feed network. FIG. 1 depicts a conventional phased-array antenna 100 having an asymmetric series-feed network. Signal 104 traveling along feed transmission line 102 is split, successively, by power splitters 110-116, and directed via branch transmission lines 120-128 to radiating elements 140-148. Branch transmission lines 120-128 are of identical length so that no phase shift is introduced by the feed network itself. Phase shifters 130-136 are operable to introduce phase shift into the signals traveling along transmission line 102.
Phase shifters 130 to 136 are disposed in feed line 102 to each individual branch line 120-128. As such, the signal entering each successive phase shifter has shifted in the preceding phase shifters. Since the phase differential required for each adjacent radiating element is .DELTA..phi., the "tuning" or "phase shifting range" for each phase shifter 120-128 is the same and has a maximum value of only 1.DELTA..phi.. In corporate-fed phased-array antennas, the phase shifters are typically located in branch lines. In such an arrangement, the signal entering each successive phase shifter has not been shifted in preceding phase shifters. As such, the total tuning range per phase shifter must increase progressively from element-to-element. For example, relative to a reference radiating element, an adjacent element is shifted by 1.DELTA..phi., which shift is provided by a first phase shifter, the next radiating element is shifted by 2.DELTA..phi., which shift is provided by a second phase shifter, and so forth. In general, the final phase shifter in a phased array using a corporate-feed network and having n radiating elements requires a tuning range of (n-1).DELTA..phi..
It will be appreciated that the required progressive increase in phase-shifting range restricts the corporate-fed phased-array to relatively few radiating elements. And, of course, each phase-shifter is different, so that manufacturing expediencies related to having identical phase-shifters, such as is possible with a series-feed implementation, are lost. It would therefore be desirable, in some embodiments, to use a series-fed phased-array antenna in preference to a corporate-fed phased-array antenna.
Series-fed phased-array antennas are not, however, without their drawbacks. In particular, phased arrays using series feed networks tend to be significantly more sensitive to design, material and manufacturing tolerances than corporate feed networks, since such tolerances are additive in series feed networks. Furthermore, the beam tilt produced by a series feed is frequency dependent. Acceptable beam-tilt variation due to such frequency dependence determines the useful frequency band ("the bandwidth") of the antenna. Moreover, there is a substantially inverse relationship between the amount of phase (which equates to electrical line length) between adjacent branch lines (e.g., 120 to 122), referred to herein as "inter-element phase," and the bandwidth of the array. Since conventional phase shifters, such as ferrites and switchable delay lines, tend to be large, their use in a series feed network may disadvantageously require an increase in inter-element line length (to accommodate them). Such additional length impacts the phased-array antenna in several ways. First, if high antenna bandwidth is required and thus small inter-element phase, the additional length required when using conventional phase shifters may be regarded as "wasted" phase since it cannot be used for the phase shifters. This ultimately limits the phase-shifting range available from the phase shifters. Second, if a fixed amount of beam steering is desired, additional phase may be required for the phase shifters so that they can provide suitable phase-shifting range to achieve the desired amount of steering.
Thus, there is a need for a steerable, series-fed phased-array antenna that keeps "wasted" inter-element phase low.